1. Field of the Invention
The present invention relates to a method and apparatus to ignite a combustible gaseous mixture, particularly a mixture of gasoline vapor and air in the combustion chamber of an internal combustion engine utilizing a spark plug.
2. Prior Art
Ignition of a fuel-air mixture in the combustion chamber of an internal combustion engine (ICE) is done by a spark plug in which a high-voltage spark, for example generated by discharge of a capacitor, is caused to discharge across a firing or spark gap of the spark plug. The capacitor, or another energy storage device such as an ignition coil itself, is charged with energy and, at a predetermined time instant which may be controlled by a computer, the capacitor or other energy storage device discharges causing the spark to flash over at the spark gap. The spark gap ignites the combustible mixture within the combustion chamber of the ICE.
Timing of the spark in relation to the combustible charge, and the position of a piston in the ICE, usually taken with reference to the top dead-center (TDC) position of the piston, is important. The spark flash over is usually caused to occur at a predetermined time instant in advance of the TDC position of the piston so that the mixture will burn, and give off energy just at and after the piston has reached TDC position, to obtain maximum efficiency from the burning mixture. For most efficient operation, it is important that the mixture should burn as rapidly as possible within the combustion chamber, and that a frontal zone of combustion, or flaming, of the combustible mixture propagates as rapidly as possible.
The electrical discharge which occurs at the spark gap of the spark plug under control of the associated ignition system is, unfortunately, not a clearly analyzable occurrence or event as, for example, an electrical square-wave pulse or the like which controls the discharge. Rudolf Maly of the Institut fur Physikalische Elektronik, Universitat Stuttgart, has suggested in numerous papers that as the spark forms, three phases can be distinguished:
(1) the breakdown phase;
(2) the arcing phase; and
(3) the glow phase.
The energy transferred in the various phases differs greatly. The formation of the respective phases depends to some extent on the geometry of the ignition electrodes, as well as on the associated circuitry connected thereto. If the ignition system provides a high-voltage pulse to the ignition electrodes, then, first, after the breakdown voltage has been exceeded, an electrically conductive plasma path will result. The currents which flow through the path between the electrodes may be very high. This occurs during phase (1), that is, the breakdown phase as the voltage falls from very high voltages (kilovolts) to voltages less than 10% of the peak.
The next phase is the arcing phase, the formation and course of which depends to some extent on the circuitry with which the spark plug is associated. The arcing phase causes current to flow in the previously generated plasma path. The voltage between the electrodes may be comparatively low or the current which flows at the beginning of the second, or arcing phase may be high. When the current during the arcing phase drops below a transition threshold, the arc will degenerate into a third, or glow phase which usually follows. The current during the third or glow phase continues to supply thermal energy to the media in the gap although much is lost to the electrodes during the relatively long period of time. The voltage is above the value of the voltage during the arcing phase.
The spark plug is stressed differentially during the respective phases. In the breakdown phase, the heat loading on the spark plug is low. In the arcing phase, the heat loading is high, and heat which is applied to the ignition electrodes of the spark plug leads to the well known erosion and deterioration of the spark plug. Relatively little erosion takes place during the glow discharge because of the low current densities and currents (&lt;100 ma) that can be sustained.
The loading conditions applied to an Otto-type ICE result in different conditions of combustible mixtures in the combustion chamber. Upon full load operation, the mixture is rich and the degree of fill of the combustion chamber is high. Igniting such a mixture does not pose any problems. An accelerated transfer of energy is not even necessarily desired. If the ICE, however, operates at low loading, or under idling condition or, even under engine braking conditions, the temperature within the combustion chamber drops rapidly and the pressure also drops. The mixture is lean, and the degree of fill of the combustion chamber of the ICE is low. Non-homogeneties of the mixture occur, and consequently, ignition of the already lean, and possibly non-homogenous and insufficiently filled, mixture may cause difficulties.
Ignition systems are known which provide a succession of spark breakdowns in order to ensure ignition of the combustible mixture in an ICE. For example, it is known to sense the composition of the combustible fuel-air mixture, and to control the number of spark flashovers, or breakdowns at the sparking electrodes of the spark plug as a function of the ratio of fuel to air in the combustible fuel-air mixture.
U.S. Pat. No. 4,653,459 to Herden teaches engine control using the relationship of the number of spark breakdowns to the fuel-air mixture composition being supplied of the engine. However, specially constructed spark plugs are required to enhance the breakdown phase. Furthermore, the higher energy impulses of these breakdown sparks may lead to undesirable RFI (radio frequency interference) emissions.
To avoid having to reconfigure the ignition components, it would be desirable to use conventional inductive discharge hardware, preferably in a distributorless configuration, with repetitive firing, and communicating the ON/OFF control for this mode from a main engine control computer. Furthermore, by truncating the length of each glow discharge to recover energy which otherwise would be lost to the spark plug electrodes and providing a number of fresh ignition sources in a turbulent mixture by repetitively firing the same spark plug gap, there exists a higher probability of igniting a lean mixture. These are some of the problems this invention overcomes.